The newest versions of Firefox block Silverlight.
Here's a fix:
Actually it is still possible to use Silverlight on Firefox 52 but you need to add a config setting:
1. Open about:config
2. Anywhere on the page, right-click, go down to New and select Boolean
3. For the name, enter plugin.load_flash_only and the boolean value, enter false
4. Restart your browser
I'm told this will only work until Firefox 53 is released in mid-April. Another solution is to use the ESR release which supports Silverlight until the next ESR release in May of 2018.